This invention relates generally to the casting of metal sheets and is particularly directed to the use of electromagnetic methods in the casting of thin metal sheets.
The use of electromagnetic methods in the casting of steel is receiving increasing attention. Electromagnetic stirring has been used for many years to improve the grain structure of cast ingots. Electromagnetic confinement methods have been commercially used for several years in the vertical casting of large ingots of aluminum and copper, and perhaps other metals. Examples of this approach can be found in U.S. Pat. Nos. 3,467,166 to Getselev et al, 3,985,179 to Goodrich et al, 4,126,175 to Getselev, 4,161,206 to Yarwood et al, 4,414,285 to Lowry et al, and 4,375,234 to Pryor. More recently, attention has been focused on near net shape casting, and in particular to the casting of thin sheets of steel. Such products are used to make auto bodies, housing siding products, appliance housings, etc.
It is advantageous to cast metal as relatively thin sheets of about 1/4 inch thickness rather than large ingots with thickness of 6 inches or more, primarily because energy and capital expense associated with hot rolling can be eliminated. The 1/4 inch thick sheets can be cold rolled after minimal or no hot rolling into the final thickness and metallurgical structure. If a thin sheet caster could be developed, it would significantly reduce the cost of sheet steel. In a conventional installation, a 10 inch thick steel slab must be manipulated by at least ten rolling machines to reduce its thickness. The rolling mill may extend as much as one-half mile and cost as much as $500 million.
Molten metal cannot be cast directly as thin sheets in a conventional casting situation where the metal is poured or forced under pressure to pass through a thin rectangular slot in a mold with walls that are water cooled. The reason is that as the metal solidifies, it sticks to the mold walls. In a conventional caster, the mold is often mechanically vibrated to loosen the sticking metal, essentially applying a tangential force to rip the frozen metal away from the mold wall. This results in a very poor quality and uneven surface when the slab leaves the caster. For thick slabs, the surface quality is not of major concern because the entire slab is mechanically worked in rolling mills. For thin sheets, the casting process just described is unusable because it would tear the sheet apart. Mainly because of this sticking problem, the use of moldless electromagnetic casting has been suggested for the production of thin metal sheets in the casting process.
Representative of such technology is U.S. Pat. No. 4,678,024 to Hull et al, the disclosure of which is hereby incorporated by reference in the present application. In this approach, electromagnetic levitation is used in the casting of thin metal sheets having a thickness on the order of 1/4 inch as the sheet leaves the caster . The electromagnetic field levitates and confines the molten sheet and prevents the metal sheet from touching a mold wall. The molten metal is cooled and solidified by radiant cooling and by directing a gas or mist stream over the relatively slow moving metal sheet.
However, moldless electromagnetic casting has a number of disadvantages. First, it is difficult to cool the molten metal sheet quickly. Both radiant cooling and convective cooling by a gas are much less efficient than conductive cooling by contact of the molten metal with a cold mold wall that is water cooled. The use of gas cooling requires the use of an inert gas, such as argon, helium, or possibly nitrogen, that does not chemically interact with the molten metal. Although analysis and experiments are not yet conclusive, it appears that inexpensive coolants such as steam or gas containing water mist may not be used in the cooling process. The amount of inert gas needed in the cooling process may render moldless casting too expensive to be used commercially.
A second disadvantage with moldless electromagnetic casting is that the use of electromagnetic fields to levitate a liquid can result in a number of magnetohydrodynamic (MHD) instabilities that can occur under different circumstances and cause the sheet to lose its shape. If any of these instabilities are allowed to occur in a casting situation, the casting process must be restarted.
The present invention overcomes the aforementioned limitations of the prior art by employing electromagnetic levitation techniques to augment the conventional mold casting approach and prevent the sticking of the solidifying metal to the mold wall. In terms of electromagnetic moldless casting, the mold wall contains the molten metal when an MHD instability occurs. In addition, because the metal is allowed to touch the mold wall, cooling can take place by relatively inexpensive water cooling. In terms of mold casting, the electromagnetic field reduces the contact pressure between the mold wall and the liquid to nearly zero, essentially eliminating or greatly reducing the sliding friction between the mold and the metal. This reduction in friction substantially reduces erosion of the mold wall, thus extending the life of the mold. Also, once the metal has even slightly solidified, a force is exerted upon the metal in a direction perpendicular to and away from the surface of the mold wall.